1. Field of the Invention
This invention relates to methods for separating a substance, usually particulates, from a fluid medium by use of a magnetic field gradient. The invention has particular application to separation of cells from biological fluids, such as blood, lymphatic fluid, urine, cell cultures, etc.
Numerous techniques are known for determining the presence or amount of an analyte in a sample, such as a biological fluid, for example, serum or urine. An in vitro assay procedure is the most common of these techniques. Many of these techniques involve competitive binding of the analyte to be determined and a labeled analog of such analyte to binding sites on a specific receptor, for example, an antibody. Many of these techniques involve a separation step where the unbound labeled analog is separated from bound labeled analog and either the bound or unbound analog is examined for the signal produced by the label. The signal is produced in relation to the amount of analyte in the sample.
Several techniques are known for separating bound and unbound fractions. For example, one may employ differential migration of the bound and the free fractions, e.g., chromatoelectrophereses, gel filtration, etc.; chemical precipitation of the bound or free fraction, e.g., by means of organic solvents, salts, acids, etc. followed by filtration or centrifugation; immunological precipitation of the hound fraction, e.g., by double antibody technique followed by filtration or centrifugation; absorption of the bound or free fraction onto selective sorbing media, e.g., charcoal, silicates, resins, etc.; magnetic separation techniques, and the like.
Magnetic separations generally fall into two general categories. There are those separations in which the material to be separated is intrinsically magnetic. On the other hand, one or more components of a mixture can be rendered magnetic by the attachment of a magnetically responsive entity. In biochemical separations, materials of interest are generally not sufficiently magnetic and thus magnetic particles bound to antibodies, lectins, and other targeting molecules have been used for isolating many of these materials. Magnetic particles targeted for specific molecules have also been used in a variety of immunoassays.
Many of the separation techniques used in immunoassays are relatively long and complicated procedures. Such procedures reduce operator efficiency, decrease thoughput, and increase the costs of tests. Other separation techniques which are rapid and simple do not adequately distinguish between the bound and free fractions and therefore are unsuited for immunoassays or can only be utilized in a limited number of tests.
2. Description of the Related Art
A method for determining the concentration of substances in biological fluids (e.g., drugs, hormones, vitamins and enzymes) wherein magnetically responsive, permeable, solid, water insoluble, micro particles are employed is disclosed in U.S. Pat. No. 4,115,534. Functional magnetic particles formed by dissolving a mucopolysaccaride such as chirpsan in acidified aqueous solution containing a mixture of ferrous chloride and ferric chloride is disclosed in U.S. Pat. No. 4,285,819. The microspheres may be employed to remove dissolved ions from waste aqueous streams by formation of chelates. U.S. Pat. No. 3,933,997 describes a solid phase radio immunoassay for digoxin where anti-digoxin antibodies are coupled to magnetically responsive particles. Small magnetic particles coated with an antibody layer are used in U.S. Pat. No. 3,970,518 to provide large and widely distributed surface area for sorting out and separating select organisms and cells from populations thereof. U.S. Pat. No. 4,018,886 discloses small magnetic particles used to provide large and widely distributed surface area for separating a select protein from a solution to enable detection thereof. The particles are coated with a protein that will interact specifically with the select protein. U.S. Pat. No. 4,070,246 describes compositions comprising stable, water insoluble coatings on substrates to which biologically active proteins can be covalently coupled so that the resulting product has the biological properties of the protein and the mechanical properties of the substrate, for example, magnetic properties of a metal support. A diagnostic method employing a mixture of normally separable protein-coated particles is discussed in U.S. Pat. No. 4,115,535. Microspheres of acrolein homopolymers and copolymer with hydrophilic comonomers such as methacrylic acid and/or hydroxyethylmethacrylate are discussed in U.S. Pat. No. 4,413,070. U.S. Pat. No. 4,452,773 discloses magnetic iron-dextran microspheres which can be covalently bonded to antibodies, enzymes and other biological molecules and used to label and separate cells and other biological particles and molecules by means of a magnetic field. Coated magnetizeable microparticles, reversible suspensions thereof, and processes relating thereto are disclosed in U.S. Pat. No. 4,454,234. A method of separating cationic from anionic beads in mixed resin beds employing a ferromagnetic material intricately incorporated with each of the ionic beads is described in U.S. Pat. No. 4,523,996. A magnetic separation method utilizing a colloid of magnetic particles is discussed in U.S. Pat. No. 4,526,681. UK Patent Application GB 2,152,664A discloses magnetic assay reagents.
An electron-dense antibody conjugate made by the covalent bonding of an iron-dextran particle to an antibody molecule is reported by Dutton, et al. (1979) Proc. Natl. Acad. Sci. 76:3392-3396. Ithakissios, et al. describes the use of protein containing magnetic microparticles in radioassays in Clin. Chem. 23:2072-2079 (1977). The separation of cells labeled with immunospecific iron dextran microspheres using high gradient magnetic chromotography is disclosed by Molday, et al. (1984) FEBS 170:232-238. In J. Immunol. Meth. 52:335-367 (1982) Molday, et al. describe an immuno specific ferromagnetic iron-dextran reagent for the labeling and magnetic separation of cells. An application of magnetic microspheres in labeling and separation of cells is also disclosed by Molday, et al. in Nature 268:437-438 (1977). A solid phase fluoroimmunoassay of human albumin and biological fluids is discussed by Nargessi, et al. (1978) Clin. Chim. Acta. 89:455-460. Nye, et al. (1976) Clin. Chim. Acta. 69:387-396 discloses a solid phase magnetic particle radioimmunoassay. Magnetic fluids are described by Rosenweig (1983) Scien. Amer. 10:136-194. Magnetic protein A microspheres and their use in a method for cell separation are disclosed by Widder, et al. (1979) Clin. Immunol. and Immunopath. 14:395-400.